US3393070A - Xerographic plate with electric field regulating layer - Google Patents

Description

July 16, 1968 c. SNELLING 3,393,070

XEROGRAPHIC PLATE WITH ELECTRIC FIELD REGULATING LAYER Filed March 1, 1965 INVENTOR. CHRISTOPHER SNELLING United States Patent 3,393,070 XEROGRAPHIC PLATE WITH ELECTRIC FIELD REGULATING LAYER Christopher Snelling, Penfield, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Mar. 1, 1965, Ser. No. 436,171 5 Claims. (Cl. 961.5)

ABSTRACT OF THE DISCLOSURE This invention relates to xerography and, more specifically, to -a xerographic plate, method and apparatus.

Xerographic ofi'ice copying has undergone an extreme- 1y large growth in the past few years. This copying technique, as originally disclosed by Carlson in US Patent 2,297,691, and as further amplified by many related patents in the field, a photoconductive insulating layer making up part of a xerographic plate is first given a uniform electrostatic charge over its entire surface to sensitize it and is then exposed to an image of actinic electromagnetic radiation such as light, X-ray or the like, which selectively drains away the charge in illuminated areas of the photoconductive insulator leaving behind charge in the nonilluminated areas to form a latent electrostatic image. This latent image is then made visible (developed) by the deposition of finely divided, electroscopic, marking material on the surface of the photoconductive insulating layer as a result of which the marking material conforms to the pattern of the latent image rendering it visible. The marking material is generally made up of a powdered mixture of a thermoplastic and a colorant and is known in the art as toner. Where the photoconductive insulator is reusable, this visible toner image is then transferred to a second surface such as a sheet of paper and fixed in place thereon to form a permanent, visible reproduction of the original. Where, on the other hand, a cheap nonreusable photoconductive insulating material is employed, the toner particles are fixed in place directly on its surface with the consequent elimination of the transfer step from the process.

Although this process has been very successful commercially, certain diflic-ulties still exist with it. For example, in charging the plate in order to sensitize it, a number of techniques have been developed and the technique which has gained widest commercial acceptance is corona charging, as more fully described in US. Patents 2,588,699 to Carlson and 2,777,957 to Walkup. Essentially, this corona discharge technique consists of spacing a filament or a plurality of filaments slightly from the surface of a xerographic plate having its conductive base grounded and applying a high potential to the filament so that an electrical corona discharge occurs between the filament and the plate, thus serving to deposit charged ions or electrons on the plate surface to raise its level of electrostatic charge with respect to ground potential. Notwithstanding the fact that corona discharge has been found to produce excellent results and is employed in one form or another in virtually every commercial electrostatic oflice copying machine, it suflFers from certain inherent difficulties by the very fact that it operates upon the principle of an ionizing electrical field discharge. Thus, for example,

the conditions of the atmosphere between the corona electrode and the xerographic plate to be sensitized can, in certain instances, make important differences in how effectively the plate is sensitized. Reduced air pressure, wide changes in relative humidity, large amounts of impurities in the air and other factors may have relatively important effects upon the level of charge which is deposited upon the plate with the charging voltage held constant. Accordingly, in order to ensure that the corona discharge deposits the desired level of charge on the plate in a copying machine, it is frequently necessary to adjust the power supply or use specialized charging devices such as screen-controlled corona discharge electrodes, as described for example in US. Patent 2,778,946 to Mayo, and even these techniques are not always entirely satisfactory. Another instance in which corona voltage controls are required is a variable speed copier using a photoconductive plate which moves past the corona charging unit at two or more different speeds.

Accordingly, it is an object of this invention to provide a novel form of xerography.

It is also an object of this invention to provide a novel xerographic plate.

A still further object of this invention is to provide a xerographic plate which is self-limiting in its charge acceptance characteristics.

An additional object of this invention is to provide an improved xerographic process.

Yet another object of this invention is to provide a novel xerographic apparatus requiring no charging power supply adjustments or controls.

The above and still further objectives may be accomplished in accordance with the present invention, generally speaking, by employing a special plate having a current-voltage characteristic on charging such that it acts as its own voltage regulator after the desired amount of charge has been applied. This type of plate is then employed with a copying device with a fixed power supply whose output voltage is above the voltage required to deposit the desired amount of charge upon the plate so that even when charging time is short or changes in relative humidity, atmospheric pressure, or the like cause drops in the output of the corona charging unit, the deposited voltage is above that which would ordinarily be required on the plate surface and the plate itself serves as the voltage regulator. The nature of the invention will be more easily understood when it is considered in conjunction with the accompanying drawings of an exemplary preferred embodiment of the invention wherein:

FIG. 1 is a side view of the improved xerographic plate according to this invention.

FIG. 2 is a graph of the current-voltage characteristics during corona charging of a conventional xerographic plate as compared with the improved xerographic plate of this invention; and,

FIG. 3 is a side sectional view of an exemplary xerographic processing apparatus employing the improved plate of this invention.

Referring now to FIG. 1, there is seen a xerographic plate generally indicated as 10 made up of supporting substrate 11, an interface layer 12 and a photoconductive insulating layer 13.

Although the use of the supporting substrate layer 11 is optional, its use may be desirable for many purposes such as to give the plate additional strength, to supply a contact with a source of electrical potential, etc. For example, the substrate layer may consist of any one of a number of materials including conductive materials such as aluminum, magnesium brass, steel, chrome, etc., or non-conductive materials such as glass, paper, plastic sheeting or the like impregnated with materials such as metals or carbon black which raise their conductivity or coated with conductive layers such as thin layers of gold, copper iodide, or the like. The use of such a conductive substrate not only provides additional structural strength to the plate, but also provides for an electrical ground plane immediately beneath the surface of the other plate layers so that the plate may be easily charged from a corona discharge electrode in accordance with the teachings of the aforementioned US. Patent 2,588,699. It is to be noted that for this purpose the conductive material need not necessarily be a material which is ordinarily thought of as an electrical conductor. Any substrate having an electrical resistance at least several orders of magnitude lower than the resistance of the illuminated photoconductor will serve this function even without a metallic coating. If on the other hand, certain other corona charging techniques such as the two-sided corona charging technique, described in U.S. Patent 2,922,883 are employed, the conductivity of the supporting substrate may be largely ignored and it may be selected based mainly on its structural properties or omitted altogether.

Interface layer 12 is a thin layer of a material which is selected primarily for its electrical properties, as described hereinafter in connection with FIG. 2. In essence, it is a material which is selected so as to impart to the plate a voltage or field regulating electrical characteristic similar to that of a voltage-regulating gas discharge tube. Any suitable voltage regulating material may be employed for this purpose. Typical materials which have been found to provide this voltage regulating characteristic include: bismuth, oxidized bismuth, tin, copper selenide, and extremely thin layers of tin oxide or aluminum oxide, etc. The tin oxide and aluminum oxide layers must be very thin in order to achieve this characteristic I.V. curve. They are produced by evaporating the underlying aluminum or tin onto the substrate under vacuum, breaking the vacuum momentarily to allow natural surface oxidation on the metal and then pumping down the system again before evaporating on the photoconductor. These oxide layers are believed to be less than about microns thick.

Overlying interface layer 12 is a photoconductive insulating layer 13. Any suitable photoconductive insulating material may be employed as layer 13. Typical photoconductive insulators include: vitreous selenium, alloys of selenium with arsenic or tellurium in the vitreous form, sintered or evaporated layers of other materials such as cadmium sulfide, cadmium selenide, etc., photoconductive insulating materials in particulate form suspended in an insulating film-forming binder material as, for example, zinc sulfide, zinc cadmium sulfide, French process zinc oxide, metal-free phthalocyanine, cadmium sulfide, cadmium selenide, zinc silicate, cadmium sulpho-selenide, etc., dispersed in an insulating film-forming binder such as an epoxy resin, a silicone resin, an alkyd resin or the like. Others include suitable blends, copolymers, terpolymers, etc. of photoconductors and non-photoconductive materials which are either copolymerizable or miscible together to form solid solutions. Typical organic photoconductive materials of this type include: polyvinylcarbazole, anthracene, polyvinylanthracene, anthraquinone, oxidiazole derivatives such as 2,5-bis-(p-aminophenyl-l), 1,3,4 oxidiazole; Z-phenylbenzoxazole; and charge transfer complexes made by complexing resins such as phenolaldehydes, epoxies, phenoxies, polycarbonates, melamines, etc. with Lewis acids such as phthalic anhydride, 2,4,7-trinitrofluorenone, metallic chlorides such as aluminum, zinc or ferric chloride; 4,4-bis(dimethylamino)benzophenone; chloranil; picric acid; 1,3,5-trinitrobenzene; l-chloroanthraquinone; bromal; 4-nitrobenzaldehyde; 4-nitr-ophenol; acetic anhydride; maleic anhydride; borontrichloride; maleic acid; cinnamic acid; benzoic acid; tartaric acid; malonic acid and mixtures thereof. It is to be noted, however, that selenium in its amorphous form and alloys of the amorphous form of selenium constitute a preferred material for photoconductive insulating layer 13, because of their extremely high quality image-making capability and relatively high light response. i

In FIG. 2, there is shown a graph of'the amount of current flow I through a xerographic plate versus the voltage on that plate V as corona charging proceeds. Curve 16, as shown on the graph, represents a current-voltage characteristic of a conventional selenium xerographic plate on an aluminum substrate while curve 17 represents the current-voltage characteristic curve of the improved plate of this invention. As can be seen from the graph, the selfregulating plate of this invention has a lower leg which very closely corresponds to that of the conventional xerographic plate until the voltage on the plate reaches a certain level where the curve turns up very sharply, rising to an almost straight vertical configuration so that increasing .plate current during charging results in little or no additional voltage being built up on the surface of the plate. The particular point at which this curve 17 begins its steep rise depends upon the particular material selected for the interface 12, the thickness of the material and the particular overlying photoconductor. Thus, for example, with copper selenide or a very thin layer of aluminum oxide overcoated with amorphous selenium the curve becomes vertical at about 30 volts per micron of selenium while it becomes vertical at about 40 volts per micron of selenium with the other interface materials listed supra.

An exemplary xerographic copying apparatus adapted to employ the xerographic plate of this invention in the form of a cylindrical drum is shown in FIG. 3. The drum, when in operation, is generally rotated at a uniform velocity in the direction indicated by the arrow in FIG. 3 so after portions of the drum periphery pass the charging unit 18 and have been uniformly charged, they come beneath a projector 19 or other means for exposing the charged plate to the image to be reproduced. Subsequent to charging and exposure, sections of the drum surface move past the developing unit, generally designated 21. This developing unit is of the eascade type which includes an outer container or cover 22 with a trough at its bottom containing a supply of developing material 23. The developing material is picked up from the bottom of the container and dumped or cascaded over the drum surface by a number of "buckets 24 on an endless driven conveyor belt 26. This development technique, which is more fully described in US. Patent 2,618,552 to Wise and 2,618,551 to Walkup, utilizes a two-element development mixture including finely divided, colored marking particles or toner and larger carrier beads. The carrier beads serve both to deagglomerate the fine toner particles for easier feeding and charge them by virtue of the relative positions of the toner and carrier material in the triboelectric series. The carrier beads with toner particles clinging to them are cascaded over the drum surface. The electrostatic field from the charge pattern on the drum pulls toner particles off the carrier beads serving to develop the image. The car-rier beads, along with any toner particles not used to develop the image, then fall back into the bottom of container 22 and the developed image moves around until it comes into contact with a copy web 27 which is pressed up against the drum surface by two idle rollers 28 so that the web moves at the same speed as the periphery of the drum. The toner in the developing mixture is periodically replenished from a toner dispenser not shown. A transfer unit 29 is placed behind the web and spaced slightly from it between rollers 28. This unit is similar in nature to the plate charging mechanism 18 in that both operate on the corona discharge principle. Both the charging device 18 and the transfer unit 29 are connected to a source of high DC potential (of the same polarity) identified as 31 and 32, respectively, and include a corona discharge Wire 33 and 34, respectively, surrounded by a conductive metal shield. In the case of charging unit 18, voltage ource 31 is preselected to be of such a magnitude that it will produce a corona discharge on the drum under almost any conditions of relative humidity and atmospheric pressure normally encountered which would tend to charge a conventional xerographic plate well above the desired voltage. This excessively high potential source is pre-set and need not be adjusted because the retained voltage on the plate is controlled by the electrical characteristics of the plate itself in such a way that any excessive current which flows through the plate during the corona discharge is drained away by the voltage regulating characteristics of the plate. For example, this voltage is generally set at from about 8,000 volts to about 10,000 volts, whereas with a conventional plate, it would be set at about 7,000 volts. In the case of the corona discharge transfer unit, charge is deposited on the back of web 27 and this charge is of the same polarity as the charge initially deposited on the drum and also opposite of polarity to the toner particles utilized in developing the drum. The discharge deposit on the back of web 27 pulls the toner particles away from the drum by overcoming the force of attraction between the particles and the charge on the drum. It should be noted at this point that many other transfer techniques can be utilized in conjunction with the invention. For example, a roller connected to a high potential source opposite in polarity to the toner particles may be placed immediately behind the copy web or the copy web itself may be adhesive to the toner particles. After transfer of the toner image to toner 27, the web moves beneath a fixing unit 36 which serves to fuse or permanently fix the toner image to web 27. In this case, a resistance heating-type fixer is illustrated. However, here again, other techniques known in the art may also be utilized including the subjection of the toner image to a solvent vapor, spraying of the toner image with an adhesive film-forming overcoating. After fixing, the web is rewound on a coil 37 for later use. After passing the transfer station, the drum continues around and moves beneath a cleaning brush 38 which prepares it for a new cycle of operation. It should be noted that this apparatus may also be operated at varying speeds by setting the corona discharge unit at a high enough voltage so the plate will be charged fully at the highest speed. Then, overcharging will not occur at the lower speeds because of self regulation by the plate.

Although the invention has been described in connection with corona charging, it is to be understood that this is exemplary only, and that the self-regulating plate may, in fact, be employed with a suitable charging technique. Other typical charging methods include friction 6 charging, induction charging, as described in U.S. Patents 2,934,649 and 2,833,930, and roller charging, as de scribed in U.S. Patent 2,934,650.

What is claimed is:

1. A xerographic plate comprising an electrically c0n ductive substrate, an electrical field regulating layer on said substrate consisting essentially of bismuth, and a photoconductive insulating layer overlaying said voltage regulating layer.

2. A xerographic plate comprising an electrically conductive substrate, an electrical field regulating layer on said substrate consisting essentially of oxidized bismuth, and a photoconductive insulating layer overlaying said voltage regulating layer.

3. A xerographic plate comprising an electrically conductive substrate, an electrical field regulating layer on said substrate consisting essentially of tin, and a photoconductive insulating layer overlaying said voltage regulating layer.

4. A xerographic plate comprising an electrically conductive substrate, an electrical field regulating layer on said substrate consisting essentially of copper selenide, and a photoconductive insulating layer overlaying said voltage regulating layer.

5. A method of uniformly charging a xerographic plate having a voltage regulating layer, said plate comprising an electrically conductive support, an electrical field regulating layer comprising a material selected from the group consisting of bismuth, oxidized bismuth, tin, and copper selenide contained on said substrate, and a photoconductive insulating layer overlaying said voltage regulating layer, said method comprising uniformly charging said plate to a charging potential in excess of a preselected voltage determined by said voltage regulating layer.

References Cited UNITED STATES PATENTS 2,863,768 12/1958 Schaffert 961.5 3,041,166 6/1962 Bardeen 96-1 3,148,057 9/1964 Raether 117-201 X 3,210,184 10/1965 Uhli-g 96-1.5 3,243,293 3/1966 Stockdale 961.5

FOREIGN PATENTS 748,340 4/1956 Great Britain.

NORMAN G. TORCHIN, Primary Examiner.

I. TRAVIS BROWN, Examiner.

C. E. VAN HORN, Assistant Examiner.

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